When a member made of metal is subjected to a hardening treatment, the carburizing treatment, in which carbon is diffused into the surface layer portion to provide a hardened layer at the surface layer portion, has been hitherto widely carried out. For example, in the case of an inner member for constructing a Birfield type constant velocity universal joint, the carburizing treatment is applied to an internal spline formed on an inner circumferential wall of a through-hole.
When the carburizing treatment is applied to such an inner member, it is necessary to avoid forming a hardened layer having an excess thickness on the internal spline, for the following reason. That is, the toughness is lowered, and thus the crack tends to appear, because the hardness of the internal spline is unnecessarily increased.
Japanese Laid-Open Patent Publication No. 2-298249 proposes application of an anti-carburization agent to an internal spline, and Japanese Laid-Open Patent Publication No. 6-2102 proposes to apply an anti-carburization treatment such as nickel plating. Japanese Laid-Open Patent Publication Nos. 4-263060 and 60-103123 disclose that an internal spline is surrounded by a seal or a spacer in order to regulate the degree of formation of the hardened layer.
When the carburization-preventive agent is used, it is necessary that the carburization-preventive agent is exfoliated and removed from the inner member after applying the carburizing treatment, which is complicated. Further, it is impossible to adjust the depth of the hardened layer.
When the seal or the spacer is used, it is necessary for the operator to manually install the seal, the spacer or the like. Therefore, it is time consuming, and hence the production efficiency is lowered.
The present applicant has proposed in Japanese Laid-Open Patent Publication No. 9-324257 that a support shaft member, which is composed of, for example, a pipe standing on a frame, is inserted into through-holes of a plurality of inner members so that the plurality of inner members are successively stacked. While maintaining a constant annular gap between the inner circumferential surfaces of the inner members and the outer circumferential surface of the support shaft member, the carburizing gas is supplied to the gap. Accordingly, the depth of the hardened layer formed on the inner circumferential surface of the inner member is substantially uniform. Further, the hardened layer formed on the inner circumferential surface is shallower than the hardened layer formed on the outer circumferential surface.
In this procedure, the end surfaces of the inner members are stacked while making close contact with each other. Therefore, the carburizing agent (gas) does not leak or leaks very little from the space between the inner members adjoining vertically. Accordingly, the carburizing agent contacts only the exposed surfaces of the stacked inner members, i.e., the two portions of the lower end surface of the lowermost inner member disposed just above the frame and the upper end surface of the inner member stacked at the uppermost position. Therefore, it is not easy to apply the carburizing treatment to the respective end surfaces of all of the stacked inner members.
Further, the hardened layer is formed on only one end surface of each of the inner members disposed at the lowermost and uppermost positions as described above. This sometimes causes the relatively large difference in the hardening strain in each of the inner members disposed at the lowermost and uppermost positions as shown in FIG. 9. In such a situation, it is difficult to maintain the dimensional accuracy of the inner member as well as that of the internal spline. In particular, when the inner member is made of high strength steel, for example, boron steel in which the crystal grain boundary is reinforced by elemental addition, a large hardening strain difference causes, because the structure sensitivity of the high strength steel is higher than that of a general case hardening steel such as SCM steel (see FIG. 9). SCM steel is an alloy steel for machine structural use which is defined by the Japanese Industrial Standards. The numerical values shown in FIG. 9 represent the results obtained on condition that the number of stacked inner members is five. In the column of the “stacked position, ” “1”, indicates the inner member disposed at the uppermost position, “3” indicates the inner member disposed at the middle position, and “5” indicates the inner member disposed at the lowermost position. “2” and “4” indicate the inner members interposed between “1” and “3” and between “3” and “5,” respectively. The hardening strain difference is dimensional difference before and after the hardening treatment at the identical portion of each of the inner members. Each inner member is measured at a position corresponding to measured positions in other inner members.